This invention relates to catheters for deployment and/or removal of prostheses in a body passageway, methods of using such catheters for prosthesis deployment and/or removal, and novel prostheses including helical stents for opening a body passageway.
The use of shape memory alloys or intermetallics and, specifically, Nitinol in the construction of medical devices is well known (Andrews et al., Minimally Invasive Therapy 4:315-318 (1995), Quin, U.S. Pat. No. 4,505,767; these and all other references cited herein are expressly incorporated by reference as if fully set forth herein in their entirety). Nitinol has been used as dental arch wire (Andreasen, U.S. Pat. No. 4,037,324), catheters (Wilson, U.S. Pat. No. 3,890,977), heart valves (Akins, U.S. Pat. No. 4,233,690), IUDs (Fannon, U.S. Pat. No. 3,620,212), bone plates (Johnson et al., U.S. Pat. No. 3,786,806), marrow nails (Baumgart, U.S. Pat. No. 4,170,990), stents (Hess, U.S. Pat. No. 5,197,978, and Mori, U.S. Pat. No. 5,466,242), vena cava filters, staples, and clips. The properties of these materials have been extensively discussed in the above-noted references and, for the sake of brevity, will not be repeated here. All of the referenced devices have characteristics which make or tend to make them impractical. Often, they require heating or cooling which is not always convenient or reliable. Loss of temperature control can cause shape change before the device is placed properly or before the surgeon is prepared for the shape change or force generation effect delivered by the device. Sometimes, in addition, when force generation is the desired effect, heat-driven shape change restrained by an element against which the force is directed (e.g., bone) results in total conversion to austenite. Austenite has a reasonably high Young""s modulus (on the order of 14 million PSI). Therefore, the residual stress or force which is generated in equilibrium initially cannot be maintained because slight changes in geometry or strain results in significant changes in stress. These slight changes in strain might be brought about by differential thermal expansion, or creep, as a result of tissue growth or accommodation in response to the force generated by the device, or by tissue atrophy.
An improvement was disclosed by Jervis, U.S. Pat. No. 4,665,906, and its progeny. This art discloses the use of pseudoelasticity to effect shape change or force generation at essentially constant temperature, in the case of medical devices, at or around body temperature. Pseudoelastic phenomena in Nitinol is brought about by the fact that stress may be used, within defined temperature and composition limits, to convert austinite to martensite. After an initial range of Hookian behavior, this austenite-to-martensite conversion occurs at essentially constant stress as loading increases. Within still further defined temperature limits, unloading causes reversion to austenite, again at essentially constant (but lower) stress. The loading/unloading sequence therefore defines a relatively flat hysteresis loop. The dimensions of this loop can be altered somewhat by alloying, or by thermal and mechanical treatment; but typical values might include a spread of from 25 to 50 KSI between the loading and unloading plateaus, and up to around 5% or more strain at essentially constant stress. At strains beyond the plateau range, the stress again rises or falls at essentially Young""s modulus, as it does initially.
Hysteresis behavior to generate shape change or force at or around constant (body) temperature is discussed in Jervis, U.S. Pat. No. 4,665,906. Shape changes resulting from this phenomenon can be significant compared to strain ranges available with conventional metals (those without austenite/martensite transformations), and forces delivered can be relatively well controlled over a wide strain range. U.S. Pat. No. 4,665,906 discusses forming the device to the final shape desired, straining the device in a direction which tends to facilitate placement into the body, restraining the device in this strained shape during insertion into or placement near the body, then releasing all or part of the device such that it returns or tends to return to the desired shape.
Among the medical devices discussed above, prostheses adapted to hold open a body passageway by expansion, such as stents, have recently been the subject of growing interest. The concept of using an expandable prosthesis to open a body passageway is discussed generally in Palmaz, U.S. Pat. No. 4,733,665, Cragg, U.S. Pat. No. 5,405,377, Gianturco, U.S. Pat. No. 5,282,824, Derbyshire, U.S. Pat. No. 5,007,926, Sigwart, U.S. Pat. No. 5,443,500, and Yachia et al., U.S. Pat. No. 5,246,445. Meanwhile, Hess, U.S. Pat. No. 5,197,978, discusses the use of a martensite nitinol stent which is deployed by inflating a balloon to expand the martensite. Building on the stent art, others have sought to employ shape memory alloys so that an expandable stent member can be implanted with thermal activation. See Kleshinski et al., International Application No. PCT/US95/03931, Regan, U.S. Pat. No. 4,795,458, Harada, U.S. Pat. No. 5,089,005, and Harada et al., U.S. Pat. No. 5,037,427.
The Regan ""458 patent teaches use of a helical Nitinol coil which expands from one diameter to another in response to application of heat. This is use of Nitinol in its oldest heat-to-change form. However, it has been observed that such use presents problems in several ways. The temperature excursions to which the body may be subjected without damage is limited. The ability to alloy Nitinol for accurate transition temperatures is difficult. The ability to control the temperature of a Nitinol device being implanted within the body is also difficult, with the danger being the shape change may occur inadvertently and potentially disastrously. The transition temperature itself is, in reality, a range of temperatures over which shape change occurs. Accordingly, there is a need to devise a stent in which use of or reliance upon temperature is eliminated as a relevant parameter in the behavior of the stent.
We have made an unexpected discovery of two ways to overcome the limitations discussed above. The first is to utilize martensitic Nitinol, and the other is to use non-reversible pseudoelastic Nitinol. In either case, work to transform the stent from one shape to another is performed within the body. Whether martensite or non-reversible pseudoelastic material is preferable depends in part on the strength required of the expanded stent. The stress/strain performance of either material provides opportunity for approaches utilizing the material in bending. In bending, the maximum strain at any point is inversely proportional to the radius of curvature at that point. When an element of constant cross-section is bent, it seldom bends uniformly (in the theoretical sense, only in pure bending). Continued loading beyond the plateau of either the martensite or the pseudoelastic material leads to a sharp rise in modulus, which means that, at the end of the plateau, the material at maximum strain is getting stronger than adjacent material which is still operating on the plateau. Gross bending of a device of uniform geometry will therefore eventually result in all elements being bent to the same strain, i.e., a circle.
For the purposes of stent deployment, there is a substantial significance associated with the ability of a material to deform with all elements eventually being bent to the same strain. For example, a small helical stent made from materials that have these properties will permit maximum unwinding which forms, after the unwinding force is removed, an enlarged helix with any cross-section being substantially circular, rather than having a kinked wire in the shape of an oval, ellipse, or other irregular geometric shape. Moreover, the enlarged helix will have a substantially uniform diameter throughout its length, meaning that the surface of the stent defines substantially a regular cylinder, rather than an irregular geometry. The unwinding of a stent to produce an enlarged stent having a substantially uniform cylindrical interior volume is a property with important physiological ramifications because a stent with irregular kinking which protrudes into the vessel lumen may create an obstruction and lead to blood flow constriction. Although described in terms of a helix, other structures which are to be deformed and must approach a predetermined shape can make similar use of this plateau-dependent behavior.
In accordance with the invention, one approach to making, for example, a vascular stent is to form a small coil for introduction purposes. When located properly in the vessel, one end of the coil is xe2x80x9cunscrewedxe2x80x9d relative to the other, increasing the coil diameter until the stent is against the vessel wall in the desired manner, and essentially at the end of the plateau strain range. The plateau shape of the stress/strain curve means that, assuming the original coil was uniform in diameter and of uniform thickness strip or wire, the final diameter of the stent would also be uniform. Simple mathematical relationships exist which relate the geometry requirements and material capabilities. End features may be added for cooperation with tools or catheters for placement utilizing torsion control. By addition of end features which could be xe2x80x9ccaughtxe2x80x9d at a later date, the stent could be xe2x80x9cscrewedxe2x80x9d together, reducing its diameter for removal. Such a stent could be contained within or coated with an outer member, e.g., Dacron(trademark), which could be left in place after the Nitinol is removed. Furthermore, if it is known ahead of time that this sort of stent is to be left in place, it may be unnecessary to utilize a PCTA catheter before placing the stent depending in part on the extent of the stenosis. The expansion of the stent would dilate the vessel as the stent is deployed.
A further advantage of using the stents disclosed herein is that the force of radial expansion is not limited to stored energy as is the case for self-expanding stents made from shape memory alloys. This factor is an important consideration when dealing with calcified, hardened regions of a vessel which are difficult to enlarge. In these regions, a greater force of expansion may be needed to achieve a given diameter lumen. Such greater force can be applied in accordance with the present invention because the application of force is completely controlled during stent deployment. This ability to control force represents yet another improvement of the present invention over self-expanding stents which exert an expansion force which is limited by the energy stored prior to deployment.